1. Field of the Invention
The present invention relates to a method of vaporizing liquid source materials for supplying such vaporized source materials to Chemical Vapor Deposition(CVD) apparatus, and more particularly in such areas as Metalorganic Chemical Vapor Deposition (MOCVD) and Atomic Layer Deposition (ALD) applications.
2. Description of the Related Art
As a part of semiconductor device manufacturing processes, it has been active in developing methods of Chemical Vapor Deposition (CVD) processes, more particularly, in such areas of Metalorganic Chemical Vapor Deposition (MOCVD) and Atomic Layer Deposition (ALD) applications, in order to form high quality films by using metalorganic source materials. However, since most of the metalorganic source materials are in either liquid state or solid state at room temperature, it is necessary to vaporize such source materials before they are supplied to the reaction chamber of the Chemical Vapor Deposition (CVD) apparatus.
As a result, several different methods of vaporizing liquid or solid source materials have been proposed, among which the simple one is to heat the source material to certain temperature until the source material evaporates.
However, this method is neither suitable for the materials which are thermally unstable at the vapor temperature nor suitable for the materials of which the vapor pressure is very low. Furthermore, in case of the liquid source material made of a solid source material by dissolving it with a solvent, the solvent vaporizes before the solute does. This means that the liquid becomes viscous easily or generate solute particles as it gets exposed in the atmosphere for a period of time. Therefore, it is difficult to maintain a steady flow of the source materials since the vapor pressure of the source material changes in time due to, for example, either high viscosity compared to the fresh liquid source material or the change of the surface area of solute particles. As a result, steady supply of vaporized source material at a constant rate becomes difficult.
Other methods of vaporizing the source materials either in liquid state or solid state have been suggested by R. H. Thring [U.S. Pat. No. 5,836,289 (1998)]. In order to increase the speed of evaporation, Thring (Porous Element Fuel Vaporizer) proposed to use a heated porous element to push liquid source material through it, and at the same time, carrier gas is also pushed through the porous element so that the mixture of the vaporized source material and the carrier gas is supplied to the desired reactor.
Li, et al [U.S. Pat. No. 5,835,677 (1998)] suggested to use an ultrasonic injection nozzle in conjunction with the method suggested by Thring aforementioned in order to improve the efficiency of the vaporization before the vaporized source material is forced into a CVD reaction chamber.
Another improved invention is suggested by J. H. Ewing [U.S. Pat. No. 5,553,188 (1996)] by using a stack of coaxially aligned, thermally conductive, thin, flat disks having different diameters, thereby allowing to form liquid films over a large surface area of the disks so that when carrier gas is forced into this large area coated with film formed by the liquid source material, the carrier gas pick up the vaporized source material more efficiently from the large surface area on the disks.
However, all the previous methods described above using either porous elements or a stack of coaxially aligned, thin, flat disks have a common problem of getting clogged with the source material deteriorated in time due to easy condensation or solidification of the liquid source material in the small and narrow areas, thereby a smooth flow of the source material into the CVD reaction chamber is disturbed by such clogging.
The object of the present invention is to resolve the problems with the prior arts described above, that is, to steadily and in desired quantities supply the necessary source materials to a CVD reaction chamber by vaporizing liquid source materials whether they are in liquid form originally or, in case of a solid source material, dissolved one by using appropriate solvent in order to make it in liquid form.
Another object of the present invention is to supply the necessary source materials in a fashion of pulsation to the CVD reaction chamber.
Another object of the present invention is to disclose an apparatus for vaporizing liquid source materials at a steady rate, whereby the flow of the vaporized liquid source material can be easily controlled, and also the clogging in the vaporizer can be eliminated.
Another object of the present invention is to disclose an apparatus for vaporizing liquid source materials in a fashion of pulsation. But, when the operation of the vaporization apparatus is ceased, it stops almost immediately so that the residual liquid source material would not flow into the CVD reaction chamber.
Yet another object of the present invention is to disclose an apparatus for vaporizing liquid source materials where the deteriorated source material in the reservoir can be easily removed in order to clear the reservoir without dumping the deteriorated source material into the reaction chamber of a CVD apparatus.
For CVD process applications aforementioned, it is desirable to supply vaporized liquid source materials to the reaction chamber of a CVD apparatus. This invention discloses a method and apparatus for vaporizing liquid source materials for CVD applications, more particularly, in such areas as, but not limited to, Metalorganic Chemical Vapor Deposition (MOCVD) and Atomic Layer Deposition (ALD) applications.
According to the present invention, a liquid source material at an equilibrium state at temperature T1 and pressure P1 is heated and pressurized to another equilibrium state at higher temperature T2 and higher pressure level P2 in such a way that the liquid source material is not vaporized during this process. In accordance with the present invention, the liquid source material is pushed through a small opening instantaneously to a lower pressure, such as P3 where P3 is lower than P2, while the temperature T2 is maintained at a similar level, in order to vaporize the liquid source material, during which period the liquid source material becomes vaporized due to sudden drop in pressure as well as due to sudden exposure to a low pressure.
This invention also discloses an apparatus and its variations for vaporizing liquid source materials using the method described above. The apparatus comprises a relatively small vent tube capped with a disk-like flat surface, called xe2x80x9cstopper,xe2x80x9d a reservoir for the liquid source material, a gas discharge tube for the vaporized source material, a heating element that heats the liquid source material to keep the temperature of the liquid source material at an appropriate temperature so the liquid material can be easily vaporized when there is a sudden pressure change, and finally a metering pump that supplies the desired liquid source material at a pressure level in the neighborhood of P2.
The present invention has an advantage of requiring only a small exposed area or the area around said vent tube with a flat cap, called xe2x80x9cstopper,xe2x80x9d for vaporizing the liquid source material, thereby reducing the possibility of being clogged up compared to the prior art where the vaporizing element becomes clogged easily due to the fact that the excessive liquid source material is turning into a viscous state around the vaporizing element whether it is a porous element or set of thin flat disks.
Several variations of vaporization apparatus are also disclosed.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description after a brief description of the drawings.